Cutting element with wear resistant crown

ABSTRACT

A cutting element adapted to be used in a rotary drill bit is made by positioning in an appropriately shaped die cavity a quantity of a mixture of tungsten carbide and 4 to 11 percent cobalt in the shape of a crown for defining an outer surface for the tip portion of the cutting element using a pressure of less than about 600 pounds per square inch; positioning in the cavity a quantity of a mixture of tungsten carbide and 12 to 17 percent cobalt sufficient to form almost all of a base portion and at least an inner part of the tip portion for the cutting element; pressing the two quantities of the mixtures together and into the die at pressures in the range of about 10 to 15 tons per square inch; and sintering the pressed insert to form the cutting element.

TECHNICAL FIELD

The present invention relates to cutting elements or inserts for use in rotary drill bits adapted to bore holes in rock, and to methods for forming such cutting elements.

BACKGROUND ART

Cutting elements or inserts for use in rotary drill bits adapted to bore holes in rock are conventionally made entirely of a sintered mixture of tungsten carbide with about 15 to 17 percent cobalt. Such cutting elements are tough and fracture resistant (since fracturing of the cutting elements during the drilling process can not be tolerated) but are not as wear resistant as is desired. It is known that a sintered mixture of tungsten carbide and about 9 to 11 percent cobalt has significantly greater wear resistance than that containing cobalt in the 15 to 17 percent range, however, such wear resistant tungsten carbide is too prone to fracture to be used to form the entire cutting element. Thus, as is described in U.S. Pat. No. 4,359,335, attempts have been made to attach wear pads of such wear resistant tungsten carbide on bodies of such tough tungsten carbide to provide the advantage of both in one cutting element. As described in U.S. Pat. No. 4,359,335, this has been done by first forming the wear pad by pressing a mixture of tungsten carbide with about 9 to 11 percent cobalt in a first die cavity at pressures of about fifteen tons per square inch, positioning that pressed, unsintered wear pad in a second die cavity, positioning a second mixture of tungsten carbide and about 15 to 16 percent cobalt in the second die over the pad, pressing the second mixture into the die at a pressure of about 15 tons per inch, and then sintering the combination to form the cutting element or insert.

Our experience with this method, however, has been that while it may adequately bond small wear pads on surfaces of tip portions of cutting elements that project from sockets in a rotary drill bit in which base portions of the cutting elements are received, the portions of the tougher tungsten carbide material around the pads will contact rock being cut or crushed and will wear away rapidly when compared to the wear pads so that support for the wear pads is lost and they break away.

When we have attempted to form tip portions for cutting elements that are completely or almost completely covered or crowned by the wear resistant tungsten carbide material using the method described in U.S. Pat. No. 4,359,335, voids have been formed at the interface between the wear resistant crown and the underlying base portion of the tough tungsten carbide material during the sintering process, and the crown has had a strong tendency to crack off during use so that the cutting element is unacceptable.

BRIEF DESCRIPTION

The present invention provides a method for making a cutting element with a body of tough tungsten carbide material and a crown of wear resistant tungsten carbide material, which cutting element has both more wear resistance at its end portion and toughness than a cutting element made only of the tough tungsten carbide material.

According to the present invention there is provided a method for forming a cutting element having a base portion adapted to be inserted in a socket in a rotary drill bit and a tip portion adapted to project from the socket. The method comprises the steps of (1) mixing a crown mixture of tungsten carbide powder and cobalt powder with the cobalt powder being in the range of four to eleven percent (preferably nine to eleven percent) of the crown mixture; (2) mixing a core mixture of tungsten carbide powder and cobalt powder with the cobalt powder being in the range of about twelve to seventealong the borehole 8, to be termed "instantaneous" with respect to a series of common equispaced logging stations d₁,d₂. . . d_(V) along the borehole 8. Potential measurements typically begin with assembly E₁ and proceed in ordered sequences through the remaining assemblies E₂,E₃, etc and end with assembly E_(2M-1). As the logging array 21 is rolled up or down the borehole one logging station say where assembly E1 is rolled down from logging station d₁ to station d₂, the collection process is repeated. Result: a series of current and potential values are systematically collected as a function of depth for later manipulation as set forth below. But note that current and potential values to be manipulated only have formation integrity if they all relate to the same stationary collection local. That is to say, values must be indexed to a particular stationary depth scan interval (equal to M logging stations as explained below) that prevents intertwining of like values of different depth scan intervals, in a manner also explained in more detail below.

FIG. 4 illustrates how systematic collection and indexing occurs during operations. For description purposes, it is assummed that the number of electrode assemblies comprising the logging array 21 has been greatly curtailed, say scaled down from the large array of FIG. 1 to a 9-electrode array comprising electrode assemblies E₁, E₂. . . E₉. Current is continuously injected by means of the current electrode (not shown) of the mid-central electrode assembly E₅. Thus, assume that the current electrode at the mid-central assembly E₅ of the 9-electrode array is activated and that absolute and difference potentials are measured at the four assemblies above the current electrode, including the current electrode assembly (i.e., at the assemblies having numbering order 1,2, . . . ,5, beginning at the shallowest assembly with respect to the earth's surface 15). Next, assume that during the second half of the collection cycle, that the same current electrode (which occupies the internally numbered 5th electrode position) remains activated and absolute and difference potentials are also measured at the four deeper assemblies, having numbering order 6,7, . . . ,9.

In associating the measured potential and current values into 5×5 matrices, it will become evident that potential quantities (both absolute and differential) collected at potential electrodes which do not lie below the current electrode will provide matrix entries on and above its diagonal, while those collected below the current electrode will provide entries below its diagonal. But because the array is continuously moving, it will be necessary to form each matrix gather from several different collection cycles.

FIG. 4 illustrates how systematic collection and indexing occurs during such operations wherein five separate collection cycles viz., cycles 1,2, . . . 5, for logging positions A,B,C,D and E are described in detail. In FIG. 4, the ordinate of the plot is in units of depth and the abscissa is in units of incremental time units 1,2 . . . 5. The spacing between the assemblies E₁,E₂, . . . ,E₉ is equal to spacing factor "a", as is the distance between adjacent logging stations d₁,d₂,d₃ . . . d₁₄. Although the array 21 is continuously moving along the borehole 8, each location A,B, . . . ,E marks a moment in time in which collection of the potential, phase and current values occurs. Note in this regard, that during collection of data in accordance with FIG. 4, the array is continuously rolled downward. Movement of the array 21 occurs because of reeling out of cable 12 via hoisting unit 16 at the earth's surface 15. The collected values are transmitted uphole via the cable 12 and thence from the hoisting unit 16 to the controller-processor circuit 17. Because of the large mass of data, indexing of the logged values is rather important and dependent upon the absolute as well as relative depth positions of the emitting current electrode as well as that of the potential measuring electrodes comprising the electrode assemblies E₁, E₂, . . . ,E₉.

For example, for measurements taken when array 21 is at position A in FIG. 4, the current electrode of electrode assembly E₅ is at depth marker d_(k) +4a coincident with logging station (d₅). For the array 21 each measuring cycle 1,2, . . . ,5 requires the collection of the following analog values: (1) eight potential difference values, (2) nine absolute potential values, (3) one current intensity value and (4) two pairs of control values related to indicating phase distortion, i.e., indicating distortion via a time difference between the current at the current electrode of the assembly E₅, and the potential at the two most remote potential electrodes. These values are transmitted uphole via cable 12 and thence at the earth's surface 15 from hoisting unit 16 to controller-processor circuit 17 for storage and manipulation in accordance with the method of the present invention.

In order to assure that addresses of the collected current and potential values are complete, the following indices are made of record, vis-a-vis the collected current and potential values, viz.: (i) by depth markers d_(k),d_(k) +a, . . . ,d_(k) +12a where the factor "a" is the incremental spacing between electrode assemblies and d_(k) is the absolute depth of the electrode assembly E₁ at the start of data collection, viz., when the arrray is positioned at position A; (ii) by consecutive numbered electrode logging stations (d₁,d₂,d₃, . . . ,d₁₃) associated with the entire logging operation as where the relative position of each station is of interest; (iii) by scan depth station number (Sd₁,Sd₂, etc.) associated with the depth of the mid-central electrode assembly, corresponding to particular matrix gathers of interest, of which seven are shown in FIG. 4, viz., d_(k) +6a, d_(k) +7a, . . . , d_(k) +12a. These values can be indexed in a number of different formats as the data is collected, typical of which being displays 46,47,48, 49, and 50, and then being re-indexed in matrix gather format as set forth in display 51. It should be further noted that the displays 46,47 . . . 50 have a further annotation tag: viz., that the depicted values forming each such display must be further indexed to indicate the depth of the current electrode of electrode assembly E₅ during each of the collection cycles 1,2,3 . . . 5 which give rise to displays 46,47 . . . 50. Such annotation system can also be carried over into re-indexed matrix gather display 51 of the impedance values associated with these measurements, as explained below.

That is to say, assume that absolute depths of the numbered logging stations are known; so that when the array 21 is located at position A then the electrode assemblies E₁,E₂, . . . ,E₉ will be associated with the internal numbering index 1,2, . . ,9 of consecutive order; hence, when the current electrode of electrode assembly E₅ is at depth d_(k) +4a and measurements at the associated electrode assemblies taken, then the absolute and differential potential values and current intensity would be indicated by the following quantities: ##EQU1##

Note with respect to the indices for the absolute potential that the first subscript relates to the internal index number of the electrode assembly at which the potential measurement occurs and the second subscript identifies the internal index number of the current electrode undergoing energization while the argument in parenthesis relates to absolute depth from say the earth's surface 15 to the position of the current electrode. In regard to last-mentioned address tag, the logging station of the current electrode viz., logging station (d₅), could also be used as a substitute since absolute depth can be later calculated.

Note that the potential differences are measured between the pairs of electrode assemblies, i.e., between electrode assemblies 1 and 2; 2 and 3; 3 and 4; etc. These values are also indexed in a similar manner as above. That is, in accordance with the following:

    ΔV.sub.i,5 (d.sub.k +4a) where

i=2,3, . . . ,9.

Note that in the above, that the first subscript relates the position of the deeper of each pair of electrode assemblies and assumes that the normalizing value for forming the difference potential value relates to the descending ordered electrode assembly. That is, the value

    ΔV.sub.2,5 (d.sub.k +4a)

indicates that the potential difference is measured between the potential electrodes of assemblies E₁ and E₂ internally numbered as 1 and 2, respectively, and that the current electrode is positioned at internal ordered numbered assembly 5, while the value

    ΔV.sub.4,5 (d.sub.k +4a)

indicates that the potential difference is measured between assemblies E₃ and E₄ internally numbered as assemblies 3 and 4 with the current emitter being associated with internal numbered assembly 5. Note that depiction of the aformentioned values as set forth above comprises entries of columns 46a and 46b of display 46. The current intensity is shown as the entry of column 46c while the time measurements T₁ (d_(k) +4a),T₉ (d_(k) +4a) associated with indicating phase distortion, if any, are set forth as the entries of column 46d.

The next step in the method in accordance with the present invention is to repeat the above-described measurements at the positions B,C, D and E in FIG. 4, viz., with the current electrode at depth locations d_(k) +5a, d_(k) +6a, d_(k) +7a and d_(k) +8a, along with the pair of control values in appropriate time coordinates so as to indicate the presence (or absence) of phase distortion, in a manner as set forth above. These values occupy entries of columns 47a,47b . . . 47d of display 47; columns 48a,48b . . . 48d of display 48; columns 49a,49b . . . 49d of display 49; and columns 50a,50b . . . 50d of display 50.

Table I, below, sets forth the measurements in tabular form for greater clarity.

                  TABLE I                                                          ______________________________________                                         C46a   C46b       C46c    C46d                                                 ______________________________________                                         V.sub.1,5 (d.sub.5)                                                                   ΔV.sub.2,5 (d.sub.5)                                                                J.sub.5 (d.sub.5)                                                                      T.sub.1 (d.sub.5)                                    V.sub.2,5 (d.sub.5)                                                                   ΔV.sub.3,5 (d.sub.5)                                                                        T.sub.9 (d.sub.5)                                                                     =   DISPLAY 46                                V.sub.3,5 (d.sub.5)                                                                   ΔV.sub.4,5 (d.sub.5)                                              V.sub.4,5 (d.sub.5)                                                                   ΔV.sub.5,5 (d.sub.5)                                              V.sub.5,5 (d.sub.5)                                                                   ΔV.sub.6,5 (d.sub.5)                                              V.sub.6,1 (d.sub.5)                                                                   ΔV.sub.7,5 (d.sub.5)                                              V.sub.7,1 (d.sub.5)                                                                   ΔV.sub.8,5 (d.sub.5)                                              V.sub.8,1 (d.sub.5)                                                                   ΔV.sub.9,5 (d.sub.5)                                              V.sub.9,1 (d.sub.5)                                                            ______________________________________                                         C47a   C47b       C47c    C47d                                                 ______________________________________                                         V.sub.1,5 (d.sub.6)                                                                   ΔV.sub.2,5 (d.sub.6)                                                                J.sub.5 (d.sub.6)                                                                      T.sub.1 (d.sub.6)                                    V.sub.2,5 (d.sub.6)                                                                   ΔV.sub.3,5 (d.sub.6)                                                                        T.sub.9 (d.sub.6)                                                                     =   DISPLAY 47                                V.sub.3,5 (d.sub.6)                                                                   ΔV.sub.4,5 (d.sub.6)                                              V.sub.4,5 (d.sub.6)                                                                   ΔV.sub.5,5 (d.sub.6)                                              V.sub. 5,5 (d.sub.6)                                                                  ΔV.sub.6,5 (d.sub.6)                                              V.sub.6,5 (d.sub.6)                                                                   ΔV.sub.7,5 (d.sub.6)                                              V.sub.7,5 (d.sub.6)                                                                   ΔV.sub.8,5 (d.sub.6)                                              V.sub.8,5 (d.sub.6)                                                                   ΔV.sub.9,5 (d.sub.6)                                              V.sub.9,5 (d.sub.6)                                                            ______________________________________                                         C48a   C48b       C48c    C48d                                                 ______________________________________                                         V.sub.1,5 (d.sub.7)                                                                   ΔV.sub.2,5 (d.sub.7)                                                                J.sub.5 (d.sub.7)                                                                      T.sub.1 (d.sub.7)                                    V.sub.2,5 (d.sub.7)                                                                   ΔV.sub.3,5 (d.sub.7)                                                                        T.sub.9 (d.sub.7)                                                                     =   DISPLAY 48                                V.sub.3,5 (d.sub.7)                                                                   ΔV.sub.4,5 (d.sub.7)                                              V.sub.4,5 (d.sub.7)                                                                   ΔV.sub.5,5 (d.sub.7)                                              V.sub.5,5 (d.sub.7)                                                                   ΔV.sub.6,5 (d.sub.7)                                              V.sub.6,5 (d.sub.7)                                                                   ΔV.sub.7,5 (d.sub.7)                                              V.sub.7,5 (d.sub.7)                                                                   ΔV.sub.8,5 (d.sub.7)                                              V.sub.8,5 (d.sub.7)                                                                   ΔV.sub.9,5 (d.sub.7)                                              V.sub.9,5 (d.sub.7)                                                            ______________________________________                                         C49a   C49b       C49c    C49d                                                 ______________________________________                                         V.sub.1,5 (d.sub.8)                                                                   ΔV.sub.2,5 (d.sub.8)                                                                J.sub.5 (d.sub.8)                                                                      T.sub.1 (d.sub.8)                                    V.sub.2,5 (d.sub.8)                                                                   Δ V.sub.3,5 (d.sub.8)                                                                       T.sub.9 (d.sub.8)                                                                     =   DISPLAY 49                                V.sub.3,5 (d.sub.8)                                                                   ΔV.sub.4,5 (d.sub.8)                                              V.sub.4,5 (d.sub.8)                                                                   ΔV.sub.5,5 (d.sub.8)                                              V.sub.5,5 (d.sub.8)                                                                   ΔV.sub.6,5 (d.sub.8)                                              V.sub.6,5 (d.sub.8)                                                                   ΔV.sub.7,5 (d.sub.8)                                              V.sub.7,5 (d.sub.8)                                                                   ΔV.sub.8,5 (d.sub.8)                                              V.sub.8,5 (d.sub.8)                                                                   ΔV.sub.9,5 (d.sub.8)                                              V.sub.9,5 (d.sub.8)                                                            ______________________________________                                         C50a   C50b       C50c    C50d                                                 ______________________________________                                         V.sub.1,5 (d.sub.9)                                                                   ΔV.sub.2,5 (d.sub.9)                                                                J.sub.5 (d.sub.9)                                                                      T.sub.1 (d.sub.9)                                    V.sub.2,5 (d.sub.9)                                                                   ΔV.sub.3,5 (d.sub.9)                                                                        T.sub.9 (d.sub.9)                                                                     =   DISPLAY 50                                V.sub.3,5 (d.sub.9)                                                                   ΔV.sub.4,5 (d.sub.9)                                              V.sub.4,5 (d.sub.9)                                                                   ΔV.sub.5,5 (d.sub.9)                                              V.sub.5,5 (d.sub.9)                                                                   ΔV.sub.6,5 (d.sub.9)                                              V.sub.6,5 (d.sub.9)                                                                   ΔV.sub.7,5 (d.sub.9)                                              V.sub.7,5 (d.sub.9)                                                                   ΔV.sub.8,5 (d.sub.9)                                              V.sub.8,5 (d.sub.9)                                                                   ΔV.sub.9,5 (d.sub.9)                                              V.sub.9,5 (d.sub.9)                                                            LEGEND:   d.sub.5 = d.sub.k + 4a; d.sub.6 = d.sub.k + 5a; d.sub.7 =                      d.sub.k + 6a;                                                                  d.sub.8 = d.sub.k + 7a; and d.sub.9 = d.sub.k                        ______________________________________                                                   + 8a                                                            

From the above-denoted measured values of potential and current intensity, the ratio of the measured values associated with the same set of electrical variables of displays 46,47,48 . . . 50 can be determined using the following indices and equations, viz. for display 46:

    Z.sub.i,5 (d.sub.k +4a)=V.sub.i,5 (d.sub.k +4a)/J.sub.5 (d.sub.k +4a), i=1,2, . . . ,9

    ΔZ.sub.i,5 (d.sub.k +4a)=ΔV.sub.i,5 (d.sub.k +4a)/J.sub.5 (d.sub.k +4a), i=2,3, . . . ,9

For the example set forth in FIG. 4, such entries are set forth in tabular form in Table II.

                  TABLE II                                                         ______________________________________                                         Z.sub.1,5 (d.sub.5)                                                                   Z.sub.6,5 (d.sub.5)                                                                       ΔZ.sub.2,5 (d.sub.5)                                                              ΔZ.sub.7,5 (d.sub.5)                          Z.sub.2,5 (d.sub.5)                                                                   Z.sub.7,5 (d.sub.5)                                                                       ΔZ.sub.3,5 (d.sub.5)                                                              ΔZ.sub.8,5 (d.sub.5)                                                             FROM                                        Z.sub.3,5 (d.sub.5)                                                                   Z.sub.8,5 (d.sub.5)                                                                       ΔZ.sub.4,5 (d.sub.5)                                                              ΔZ.sub.9,5 (d.sub.5)                                                             DISPLAY 46                                  Z.sub.4,5 (d.sub.5)                                                                   Z.sub.9,5 (d.sub.5)                                                                       ΔZ.sub.5,5 (d.sub.5)                                   Z.sub.5,5 (d.sub.5)                                                                              ΔZ.sub.6,5 (d.sub.5)                                   Z.sub.1,5 (d.sub.6)                                                                   Z.sub.6,5 (d.sub.6)                                                                       ΔZ.sub.2,5 (d.sub.6)                                                              ΔZ.sub.7,5 (d.sub.6)                          Z.sub.2,5 (d.sub.6)                                                                   Z.sub.7,5 (d.sub.6)                                                                       ΔZ.sub.3,5 (d.sub.6)                                                              ΔZ.sub.8,5 (d.sub.6)                                                             FROM                                        Z.sub.3,5 (d.sub.6)                                                                   Z.sub.8,5 (d.sub.6)                                                                       ΔZ.sub.4,5 (d.sub.6)                                                              ΔZ.sub.9,5 (d.sub.6)                                                             DISPLAY 47                                  Z.sub.4,5 (d.sub.6)                                                                   Z.sub.9,5 (d.sub.6)                                                                       ΔZ.sub.5,5 (d.sub.6)                                   Z.sub.5,5 (d.sub.6)                                                                              ΔZ.sub.6,5 (d.sub.6)                                   Z.sub.1,5 (d.sub.7)                                                                   Z.sub.6,5 (d.sub.7)                                                                       ΔZ.sub.2,5 (d.sub.7)                                                              ΔZ.sub.7,5 (d.sub.7)                          Z.sub.2,5 (d.sub.7)                                                                   Z.sub.7,5 (d.sub.7)                                                                       ΔZ.sub.3,5 (d.sub.7)                                                              ΔZ.sub.8,5 (d.sub.7)                                                             FROM                                        Z.sub.3,5 (d.sub.7)                                                                   Z.sub.8,5 (d.sub.7)                                                                       ΔZ.sub.4,5 (d.sub.7)                                                              ΔZ.sub.9,5 (d.sub.7)                                                             DISPLAY 48                                  Z.sub.4,5 (d.sub.7)                                                                   Z.sub.9,5 (d.sub.7)                                                                       ΔZ.sub.5,5 (d.sub.7)                                   Z.sub.5,5 (d.sub.7)                                                                              ΔZ.sub.6,5 (d.sub.7)                                   Z.sub.1,5 (d.sub.8)                                                                   Z.sub.6,5 (d.sub.8)                                                                       ΔZ.sub.2,5 (d.sub.8)                                                              ΔZ.sub.7,5 (d.sub.8)                          Z.sub.2,5 (d.sub.8)                                                                   Z.sub.7,5 (d.sub.8)                                                                       ΔZ.sub.3,5 (d.sub.8)                                                              ΔZ.sub.8,5 (d.sub.8)                                                             FROM                                        Z.sub.3,5 (d.sub.8)                                                                   Z.sub.8,5 (d.sub.8)                                                                       ΔZ.sub.4,5 (d.sub.8)                                                              ΔZ.sub.9,5 (d.sub.8)                                                             DISPLAY 49                                  Z.sub.4,5 (d.sub.8)                                                                   Z.sub.9,5 (d.sub.8)                                                                       ΔZ.sub.5,5 (d.sub.8)                                   Z.sub.5,5 (d.sub.8)                                                                              ΔZ.sub.6,5 (d.sub.8)                                   Z.sub.1,5 (d.sub. 9)                                                                  Z.sub.6,5 (d.sub.9)                                                                       ΔZ.sub.2,5 (d.sub.9)                                                              ΔZ.sub.7,5 (d.sub.9)                          Z.sub.2,5 (d.sub.9)                                                                   Z.sub.7,5 (d.sub.9)                                                                       ΔZ.sub.3,5 (d.sub.9)                                                              ΔZ.sub.8,5 (d.sub.9)                                                             FROM                                        Z.sub.3,5 (d.sub.9)                                                                   Z.sub.8,5 (d.sub.9)                                                                       ΔZ.sub.4,5 (d.sub.9)                                                              ΔZ.sub.9,5 (d.sub.9)                                                             DISPLAY 50                                  Z.sub.4,5 (d.sub.9)                                                                   Z.sub.9,5 (d.sub.9)                                                                       ΔZ.sub.5,5 (d.sub.9)                                   Z.sub.5,5 (d.sub.9)                                                                              ΔZ.sub.6,5 (d.sub.9)                                   LEGEND:   d.sub.5 = d.sub.k + 4a; d.sub.6 = d.sub.k + 5a; d.sub.7 =                      d.sub.k + 6a;                                                                  d.sub.8 = d.sub.k + 7a; d.sub.9 = d.sub.k + 8a                       ______________________________________                                    

Thus in general, for a fixed matrix having M×M entries (the largest internally numbered electrode assembly having index 2M-1, where M=2N+1=5 in the above example) the gather about depth marker d_(k) +(M+N-1)a will be constructed from a subset of the following quantities: ##EQU2## In the above quantities, the index "r" represents a numerical display index identifying particular records involved with generation of the gather for each current activation per display. For example for the displays 46,47 . . . 50 to form matrix gather 51 of FIG. 4, it is seen that until there are 5 displays, there are insufficient records to generate a gather.

Thereafter, the above results can be re-indexed in matrix gather format to generate the display 51 as previously mentioned. Note in this regard that the matrix entries set forth in the display 51 preserve the one-to-one relationship of the current and potential values collected with the logging array 21 at the different logging positions in FIG. 4. These entries are set forth in tabular form in Table III and have been annotated for discussion purposes in Table II. In comparing the entries of Tables II and III, note that the scan depth (Sd₁) of the depicted matrix gather is coincident with depth marker (d_(k) +6a) that is two depth markers below where the mid-central assembly was initially positioned as collection occurred (i.e., at cycle 1), while the next in time scan depth (Sd₂) is at a depth of d_(k) +7a which is one logging station below Sd₁.

                                      TABLE III                                    __________________________________________________________________________            C1    C2    C3    C4    C5                                              __________________________________________________________________________     ΔZ(Sd.sub.1) =                                                                    Z.sub.5,5 (d.sub.5)                                                                  Z.sub.4,5 (d.sub.6)                                        

                                10.0       1            -          15                                          20.0       1            -          16                                          ______________________________________                                    

After a program for opening the gate to a half-open position is completed as shown in Table 1, parameters T_(i), P_(i) and F_(i) respectively representing the pulse period, the number of pulses and the direction of rotation in the ith period of operation are stored in the computer 20 through the input-output means 24. The computer 20 then asks the user, advantageously by writing questions on the display means (not shown), how wide the gate should be opened (for example, 100% for opening completely and 50% for opening to the half-open position) and how long it should take to open and close the gate (for example, 100% for a cycle with the aforementioned period of 300 ms as shown in FIG. 5). If the user responds with 100% to both these questions (that is, the gate is fully opened and closed within 300 ms), the computer 20 creates a working program on the basis of Table 1 by multiplying P_(i) Table 1 by 2 and dividing T_(i) of Table 1 by 2. If the user wants to open the gate 75% with the same period of time, T_(i) of Table 1 are likewise multiplied by 3/4 and P_(i) by 4/3. If it is desired to double the period and the user responds with 200% to the second question, the computer 20 multiplies T_(i) by 2 with P_(i) remaining unchanged. After the working program is thus created, incorporating the input by the user, the computer 20 stores it in its memory means and causes the driver means 16 to operate the step motor 12 according to this stored working program.

Operation of the program shown by Table 1 by the computer 20 is explained next by way of a flow chart shown in FIG. 6. After dummy indices P and i, representing the second and fourth columns of Table 1, are set to 0 and 1, respectively (n1), it is examined if the end of the program has been reached (n2), the end of the program being represented by i=17 because Table 1 contains 16 periods (i=1-16). Corresponding to the first line of Table 1 (i=1), the computer 20 reads the entry T₁ =0.67 ms and starts the timer 22 after setting this value therein (n3). When the specified time period has been counted by the timer 22 (YES in n4), the sign of F₁ is checked (n5). Finding that the motor 12 must be rotated in the positive direction in the first period, the computer 20 prepares a pulse for causing the stepping motor 12 to rotate by one step in the positive direction and transmits it to the driver means 16 (n6). It also counts the number of times such a pulse is transmitted (n7) until the counted value reaches the desired number of pulses for this period (P₁ =89) according to the program (n8). When the dummy index (counter) P reaches 89, it is the end of the first period (i=1) and the program enters the second period represented by the second line of Table 1 by resetting P and increasing i by 1 (n9).

Thereafter, the same routine is followed for each of the subsequent periods. In the sixth period where F₆ =0 (YES in n10), it goes without saying that no pulse is transmitted to the driver means 16. In the periods where F_(i) is negative, it also goes without saying that pulses are created for causing the stepping motor 12 to rotate by one step in the negative direction (n11). By the end of the sixteenth period (YES in n2), the hopper gate has been opened and closed with the desired motion characteristics as shown in FIG. 5.

There are many advantageous ways in which the present invention can be applied. Since the stepping motor according to the present invention can be controlled by a variety of programs, the gate can be opened as quickly as possible, for example, to increase the speed of discharge. Alternatively, the gate can be exponentially accelerated and decelerated at the beginning and end so as to reduce the noise of impact caused by sudden movements.

When sticky articles are being handled by the hopper, as another example, it may be found advantageous to cause the gate to execute a vibratory motion with a small amplitude over a predetermined short period of time after the gate has been opened. This can be accomplished by reversing the direction of rotation of the step motor a predetermined number of times to move the gate back and forth.

FIG. 8 shows an example of combinational weighing system 210 which can utilize the method and apparatus of the present invention. Combinational weighing means weighing articles by a plurality of weighing devices, performing arithmetic operations for combinations of measured weight values and then selecting a combination according to a predetermined criterion. The major features of combinational weighing are great accuracy and high throughput. Many types of combinational weighing systems have been manufactured and sold by the present assignee corporation. With reference to FIG. 8, the articles to be weighed are transported by a conveyor means (not shown) and dropped onto a dispersion table 212 which is a circular table with a lightly inclined conical top surface so that the articles dropped thereonto from the conveyor means can be made to disperse uniformly in radial directions. A plurality of feed troughs 213 each with an article receiving end and an article delivering end are arranged in a circular formation around the dispersion table 212 with their article receiving ends adjacent thereto. Both the dispersion table 212 and the feed troughs 213 are supported on a system housing 215 preferably through individual vibration-causing means (not shown) which serve to cause vibrational motion of the articles thereon. The feed troughs 213 are disposed radially and serve to deliver the articles to be weighed into the individual article batch handling units associated therewith. Each article batch handling unit includes a pool hopper 217 serving to receive an article batch from the feed trough 213 associated with the article batch handling unit to which it belongs and to discharge the same article batch into a weigh hopper 218 belonging to the same article batch handling unit and situated immediately therebelow. Each weigh hopper 218 is connected to a weighing device (not shown) such as a load cell and serves momentarily to hold the article batch received from the pool hopper 217 thereabove. The weight values measured by the load cells are electrically transmitted to a control unit (not shown) which includes a computer. Control units for combinational weighing systems have been disclosed, for example, in U.S. Pat. Nos. 4,396,078, 4,399,880 and 4,491,189. Computer algorithms for selecting a combination have also been disclosed in these references and incorporated in products produced and sold by the assignee corporation. The lower part of the system 210 is comprised of a chute assembly. According to the embodiment shown in FIG. 8, the chute assembly includes a funnel-shaped outer chute 220 coaxially surrounding a funnel-shaped inner chute 221 in such a way that they form two separate discharge routes. At the bottom end, the outer chute 220 is divided into two separate passages where it is connected to left-hand and right-hand timing hoppers 224 which are, in turn, connected to a lower chute 225 so that the articles discharged into the outer chute 220 join together. At the bottom of the inner chute 221 is provided another timing hopper 227 which is connected to a second lower chute 228. The system 210 described above, however, is but one example of combinational weighing system to which the present invention can be applied and hence is not intended to limit the scope of the invention.

In the case of such a combinational weighing system having many article batch handling units, each including one or more hoppers, one program may be established for all hoppers or the system may be so designed that the hoppers can be programmed individually. In the latter case, the hoppers are identified, for example, by different identification numbers and the user is requested to specify an identification number in addition to how wide its gate should be opened and how long it should take to open and close the gate as described above. It is particularly advantageous to be able to operate different hoppers by different programs in the case of a combinational weighing system of the type described in U.S. Pat. Nos. 3,939,928 and 4,494,619, which is based on the principle that articles can be charged more efficiently by being divided into two groups than by being vibrated and pressed in one batch. Such a system would have two weighing apparatus, one weighing and discharging a number of articles which have a total weight smaller than a target weight and the other thereafter correcting the weight of the remaining articles. Since articles are generally supplied to these two apparatus and discharged therefrom in different amounts and at different speeds, the overall efficiency of the system can be improved by specifying individually optimum modes of operation for these hoppers.

In the case of a weigh hopper including a weighing means such as a load cell to measure the weight of its contents, furthermore, a lever of the like for actually communicating the gate-opening force from the stepping motor 12 to the hopper gate must be separated from the hopper during the weighing process, or when the gate is completely closed as illustrated, for example, in FIG. 6 of the aforementioned U.S. Pat. No. 4,520,884. A predetermined clearance, therefore, is left between such a lever and a roller or the like on which the lever applies a force to open the gate. For this reason, there is generally a delay between the time when a command signal is received for opening the gate and the time at which the lever comes in contact with the roller and the gate actually begins to open. With conventional gate-opening mechanisms, this delay in time has been one of the problems in the attempt to reduce the overall weighing time. This delay in the response, however, can be eliminated as follows by the method and apparatus of the present invention. Although there is left a predetermined clearance between the lever and the roller as explained above while the gate is closed during a weighing process, the stepping motor is driven by a very small number of steps as soon as the weighing is completed so that the lever moves and comes to a point where it lightly touches the roller without actually opening the gate. This can be done because there is usually a play of about 3 mm in the linkage mechanism for communicating force. After the measured weight is used in combinational computation and the hopper is selected in the combination, the stepping motor associated with the hopper can start opening its gate without any delay because the lever is already in contact with the roller. When the gate is closed, the stepping motor is rotated in the reverse direction to separate the lever from the roller by a predetermined distance such that the hopper is ready for the next weighing operation.

Such control can be effected easily by starting with an action curve of the type illustrated in FIG. 5 and then preparing a program as illustrated in Table 1. FIG. 7 is an illustrative flow chart for the aforementioned operation of a weigh hopper. While the weighing takes place (NO in n21), the control system is engaged in other operations (n22) but as soon as the weighing is completed, the stepping motor is rotated by a predetermined number of steps until the lever touches the roller without opening the gate (n23). Thereafter, the system waits for a drive signal (n24). If the hopper has been selected as a result of combinational computation and a discharge signal is received (YES in n25), the gate is opened to discharge the articles contained therein and then is closed according to the program shown in FIG. 3 or Table 1 (n26). If the hopper was not selected and did not receive a discharge signal (NO in n25), the stepping motor is rotated in the reverse direction to separate the lever from the roller (n27). The flow chart of FIG. 4, however, is merely intended as an example of programming and is not intended to limit the present invention.

Additional advantages of the present invention include adjustability of the maximum angle to which each gate is opened, depending on the amount of articles which are contained in the hopper. As a result, the speed of weighing can be freely changed according to the amount of supplied articles. Since the motion characteristics of the gate can be easily changed through an input device, there is increased freedom in the design and all kinds of articles can be supplied and discharged in manners best suited for their individual characteristics. When sticky articles are being handled, as mentioned above, the gate can be made to vibrate after it is opened such that errors in measurement caused by articles which failed to be discharged can be usually eliminated.

The present invention is conveniently utilized in the type of combinational weighing programs where different hoppers discharge at different times as disclosed in U.S. Pat. No. 4,460,880, or where the article supplying section is separated into partitions such that different kinds of articles to be weighed are supplied as disclosed in U.S. Pat. No. 4,549,617. Since each action mode of the gate can be stored in memory means as data, many programs representing different action modes, each suited to a particular type of articles to be weighed, can be stored such that the user can select the best mode of action, depending on the conditions such as the target weight, and call the corresponding program to operate the hopper gates as shown in U.S. Pat. No. 4,553,616.

The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching. For example, the present invention can be applied to a hopper with one gate or with two gates although the description of the control unit given above was for the case of a single-gate hopper for the sake of convenience. The timer 22 may be an external timer rather than an internal software timer. If it is an external timer, Step n4 of FIG. 6 will be replaced by an interrupt. Such modifications and variations which may be apparent to a person skilled in the art are intended to be included within the scope of this invention. 

What is claimed is:
 1. A method of controlling the motion of a hopper gate actuated by a stepping motor, said method comprising the steps ofselecting a mode of opening and closing motion of said hopper gate in terms of positions thereof at selected times, forming a program by determining a sequence of pulses to be applied to said stepping motor so as to execute said mode of motion by said hopper gate, and transmitting pulses to said stepping motor according to said program.
 2. The method of claim 1 further comprising the step of storing said program in a memory means.
 3. The method of claim 1 wherein said program causes said gate to gradually accelerate to a maximum speed and to gradually decelerate to stop said gate.
 4. The method of claim 1 wherein said program causes said gate to vibrate a predetermined number of times after opening.
 5. An apparatus for controlling the motion of a hopper gate comprisingcontrol means for sequentially transmitting a series of pulse signals according to a program, and driving means for moving said hopper gate by an angle and in a direction in response to each of said pulse signals.
 6. The apparatus of claim 5 wherein said driving means include a stepping motor.
 7. The apparatus of claim 5 wherein said control means include a computer.
 8. The apparatus of claim 5 wherein said control means include memory means which store said program.
 9. The apparatus of claim 5 wherein said program causes said gate to gradually accelerate to a maximum speed and to gradually decelerate to stop said gate.
 10. The apparatus of claim 5 wherein said program causes said gate to vibrate a predetermined number of times after opening.
 11. The apparatus of claim 5 wherein said control means include an input device through which said program can be modified.
 12. The apparatus of claim 6 wherein said stepping motor has a shaft with a cam secured thereonto, said driving means further including a cam follower, which is disposed so as to be pushed by said cam, linkage means connected in motion-communicating relationship to said cam follower and to said gate, and spring means applying biasing force to keep said gate closed.
 13. In a combinational weighing system comprisinga plurality of article batch handling units each serving to receive an article batch, to output a weight value signal indicative of the weight of said article batch, and to discharge said article batch in response to a discharge signal, each article batch handling unit including one or more hoppers with gates, and a control means serving to periodically carryout combinational computation on the basis of inputted weight values, to thereby select at least one combination of article batch handling units and to output discharge signals to said selected article batch handling units, the improvement wherein said control means controls the motion of said gate by sequentially transmitting a series of pulse signals according to a preset program, and wherein each of said gates is moved by an angle and in a direction in response to each of said pulse signals.
 14. The combinational weighing system of claim 13 wherein each of said gates is operated by a stepping motor.
 15. The combinational weighing system of claim 13 wherein said control means include memory means which store said program.
 16. The combinational weighing system of claim 13 wherein said program operates each of said gates individually. 